1. Field of the Invention
This invention relates to a method for initializing a magneto-optical disk. This invention particularly relates to a method for initializing a magneto-optical disk wherein all parts of a magnetic layer constituting a recording layer become magnetized in the vertical direction.
2. Description of the Prior Art
Magneto-optical disks enable information to be recorded at a high density with a laser beam. When information is recorded on a magneto-optical disk, a laser beam is irradiated to the recording layer of the magneto-optical disk while a magnetic field is applied in a predetermined direction thereto, which results in the recording layer being locally magnetized in a desirable direction.
In general, the recording layer is formed on a substrate, which may be constituted of a polycarbonate or the like, and a layer of an amorphous alloy of a rare earth metal and a transition metal, which layer is overlaid on the substrate with a layer forming technique, such as sputtering. The recording layer is formed so that its easily magnetizable axis (C axis) is vertical with respect to the surface of the substrate. However, all parts of the recording layer are not originally magnetized vertically; rather, they are magnetized in random directions. Therefore, before the magneto-optical disk is used to record information, it must be initialized so that all parts of the recording layer will be magnetized in the vertical direction.
When a magneto-optical disk is initialized, it is located in a magnetic field having an intensity higher than the coercive force (Hc) of the magnetic layer, which constitutes the recording layer. However, because the coercive force of the magnetic layer is generally as high as several killooersteds, and sometimes even higher than 10 kOe, it is not easy to initialize a magneto-optical disk with ordinary methods, when a static magnetic field is applied to the magneto-optical disk. Therefore, a method has been proposed wherein a laser beam is irradiated to a magneto-optical disk, which is rotating, in order to heat the recording layer of the magneto-optical disk to the Curie temperature and thereby decrease the coercive force of the recording layer, after which a weak static magnetic field is applied to the magneto-optical disk in order to cause all parts of the recording layer to be magnetized in a single direction. The proposed method accurately initializes a magneto-optical disk with a weak static magnetic field. However, the proposed method is not suitable for mass-production of magneto-optical disks because the laser beam must be irradiated to the magneto-optical disk over its whole track length, and therefore as much as 10 minutes of processing time is required for each magneto-optical disk even when the magneto-optical disk is rotated at a speed of 1,800 rpm with a disk drive unit.
In order to cope with the aforesaid problems, a technique wherein a radio-frequency induction heating process is utilized has been disclosed in, for example, Japanese Unexamined Patent Publication No. 63(1988)-213138. With the disclosed technique, a plurality of stacked magneto-optical disks are subjected to a radio-frequency induction heating process in order to heat only the recording layers, which are constituted of electrically conductive materials, to a temperature near the Curie temperature, thereby decreasing the coercive force (Hc) of the recording layers. While the magneto-optical disks are in this condition, a predetermined static magnetic field is applied to them, after which they are cooled to normal temperatures. In this way all parts of the recording layers are magnetized in a single direction.
However, a recording layer is very thin (approximately 1,000 A). Therefore, with the aforesaid technique wherein a radio-frequency induction heating process is utilized, a substantially large amount of Joule heat cannot uniformly be generated over the whole area of each recording layer, and initialization cannot be achieved accurately. Also, heat from the recording layer is transferred to the substrate and layers other than the recording layer, which causes these layers of the magneto-optical disk to deform and change their optical characteristics. Moreover, the required equipment is expensive because of the radio-frequency power source and the like.